When rock or mineral is excavated in underground mining operations, the static stress distribution around the excavation is redistributed. The vertical stresses on the roof and floor and the lateral stresses on the side (pillar) walls or ribs are removed, and to maintain equilibrium, the rock subsides, the floor heaves, and the side walls bulge out. These initial movements are usually very small and take place during the actual excavation operations. Consequently, they are rarely, if ever, measured. Subsequent movements of the excavation are brought about by a combination of two factors: workings in adjacent strata, which causes a further redistribution of the stresses; and creep and fracture of the rock in the roof, floor, and sides (pillars) of the excavation.
The stability of an excavation is usually measured in terms of the magnitude and nature of the movements that take place around it. While there can often be significant movement in the floor and side walls, from the point of view of safety the roof bed movement is usually the one of most concern. Roof falls are almost invariably preceded by measurable displacements in the vertical direction. The fall itself merely represents the final stage in which the falling material is physically separated from its surroundings. It has been established by tests over many years that the magnitude and rate of change of roof bed movements can provide a reliable indication of roof stability.
There are two basic measurements made on mine roofs to determine stability: roof to floor movements (convergence) and the opening of fracture and bedding planes within the roof strata (roof bed separation). The instruments and techniques currently in use in the mining industry to measure roof movements have included extensometers set in bore holes. Measurements with the extensometers have been made on site using micrometers or simple scales. However, where detailed investigations require a continuous monitoring, a remote reading location away from mining traffic is necessary. Where only day-to-day measurements are needed, it is important to keep both equipment and data interpretation as simple as possible.
It has been suggested that a linear distance measuring device such as disclosed in U.S. Pat. No. 3,898,555 to Tellerman could be used to measure roof movements in a mine. However, it was found that the readout circuit of the Tellerman patent was not suitable for use in a mine because it was tuned to each transducer, consumed a relatively large amount of power, and was not compatible with cabling over excessive distances. Other position indicating devices are disclosed in U.S. Pat. Nos. 4,028,619 to Edwards; No. 3,423,673 to Bailey et al; and No. 4,071,818 to Krisst. However, there has not been disclosed in the prior art a suitable readout circuitry for use in a mine. In order to be used in a mine, the readout circuitry must be portable. In addition, the readout circuit should be battery-operated and approved under current standards of the Mining Safety and Health Administration for devices used in methane air mixtures. The work which resulted in the invention disclosed and claimed herein was done under contract with the U.S. Bureau of Mines by Ivor Hawkes Associates, Lebanon, New Hampshire. The final contract report relating to this work was dated July 1978 and first made available to the public on Sept. 3, 1980.